1. Field of the Invention
The present disclosure relates to tires. More particularly, it relates to a non-pneumatic rotationally molded tire for center pivot irrigations systems.
2. Related Art
In center pivot irrigation systems, water from a centrally located source (e.g., a well) is directed along a long boom pipe to the field. The boom pivots around the center point on spaced apart towers, which support the boom. The towers are generally supported by large pneumatic rubber tires similar to conventional tractor tires. The weight supported by each tower is substantial causing the tires to sink deeply into the ground. As the tires roll across muddy soil, the tires tend to push mud to the sides and front of the tire. The accumulated mud pushed to the front of the tire can eventually build up to the point that the tire can no longer advance. This essentially shuts down irrigation until the tire can be pulled out of the rut. Rubber tires are also subject to damage or wear causing a flat tire, which also shuts down the system until the flat tire is repaired or replaced.
Rotational molding is a method of manufacture for primarily hollow or partial shell shaped plastic objects. Rotational molding (also known as “rotomolding”) is typically used to produce products that are too large or complicated to be produced by other molding techniques, such as thermoforming or injection molding. Some products commonly formed by rotational molding include garbage can, fuel tanks, storage tanks, casks, and toys.
Rotational molding generally utilizes a shell mold having a cavity bounded on five sides by the mold. The sixth side of the cavity is formed by a cover attached to one or more of the adjacent sides. There are typically four steps, or stages, in a rotational molding process: loading, molding (or curing), cooling, and unloading.
When the cover to the mold is open, a powdered plastic resin, which is typically colored, is placed into the mold cavity. The cover is then sealed and the mold is placed in a heated environment in which it is rotated about two axes. The heat causes the plastic to melt against the heated inside surface of the mold. The mold continues to rotate during heating, and the plastic gradually becomes distributed evenly on the mold walls through gravitational force. While continuing to rotate, the mold is moved out of the oven to a cooling chamber, where the mold and the plastic are cooled to the point that the molded object will retain its shape. During cooling, the molded object typically shrinks away from the mold. Once the part is sufficiently cooled, it is removed from the mold and trimmed and cut to form the final part or product.